1. Field of the Invention
The present invention relates to a wheel support bearing assembly for supporting a vehicle wheel of an automotive vehicle such as a motor vehicle rotatably relative to the vehicle body and, also, to a method of manufacturing the same. More particularly, the present invention relates to the wheel support bearing assembly of a kind, in which the strength and the durability of a hub axle unit under a rotational bending condition are increased, and, also, to the method of manufacturing such wheel support bearing assembly.
2. Description of the Prior Art
A wheel support bearing assembly for the motor vehicle is currently available in two types, one for supporting a vehicle driven wheel and the other for supporting a vehicle drive wheel, and there is a variety of designs that can be selected depending on the particular purpose of use. By way of example, a conventional wheel support bearing assembly designed to support the vehicle drive wheel such as shown in FIG. 5 includes, as major components, an inner member 50 made up of an hub unit 51 and an inner ring 52, double row rolling elements 53 and 54, an outer member 55 and a constant velocity universal joint 56 for transmitting an engine power to the hub unit 51. In this type of the bearing assembly for the support of the vehicle drive wheel, a medium carbon steel for application in mechanical structures, such as S53C, is largely employed as a material for the hub unit 51, by which the vehicle wheel (not shown) and a brake rotor 57 are supported, due to the easiness of forging, cutting performance, thermal refinement characteristic or economical efficiency. Reduction in size and weight of the wheel support bearing assembly of the kind discussed above, as well as that of the hub unit 51, contributes considerably to improvement in mileage and traveling stability of the motor vehicle and, therefore, attempts have been made in progress to reduce the wall thickness of a wheel mounting flange 58 integral with the hub unit 51 and/or to employ such wheel mounting flange 58 with a plurality of ribs. On the other hand, along with improvement in performance of the motor vehicle, the load on the bearing assembly has increased. However, the mechanical strength of the hub unit 51 itself is coming close to the fatigue limit of the medium carbon steel that is used as a material for the hub unit 51, and, accordingly, it has now come to be difficult to achieve a further reduction in size and weight and a further increase of the durability.
Particularly in the hub unit 51, where the wall thickness of the wheel mounting flange 58 is to be reduced for the purpose of reducing the weight thereof, rotational bending stresses tend to concentrate on an outboard root portion of the wheel mounting flange 58, that is, a corner area 61 ranging from a brake rotor mounting surface 59 to a cylindrical pilot portion 60 and, accordingly, countermeasure against the stress concentration is required to be taken. In view of this, it may be contemplated to increase the dimension of the corner area 61, that is, the radius of curvature of that corner rear 61 to thereby relieve the generation of the stresses, but this in turn leads to a problem associated with interference with the brake rotor 57 that is fitted to the wheel mounting flange 58. The hub unit for the support of the vehicle drive wheel has a center bore defined therein and also has serrations 64 formed in an inner peripheral surface of the center bore for engagement with complemental serrations formed in a stem portion of the constant velocity universal joint 56. The rotational bending stresses tend to concentrate on a radially inwardly stepped area of the hub unit, on which the inner ring 52 is mounted in contact therewith, and, also, on the serrations 64 on the inner peripheral surface of the center bore and, accordingly, a countermeasure to increase the durability of the serrations 64, which have not been subjected to the induction hardening, is often required.
In view of the foregoing background, the applicant or assignee of the present invention has suggested a wheel support bearing assembly of a type, in which the attempts were made to, without altering the shape and the dimensions of the wheel mounting flange 58, reduce the weight and increase the strength of the hub unit 51. This wheel support bearing assembly is illustrated in FIG. 4 and includes a surface hardened layer 62 formed by means of an induction hardening technique in the corner area 61 of the wheel mounting flange 58 in the hub unit 51. In this way, the corner area 61 of the wheel mounting flange 58, which has hitherto been considered the most fragile area susceptible to the rotational bending fatigue, can be reinforced to a high strength, resulting in increase of the durability of the hub unit 51.
Other than that area, that is, a seal land area a, where a sealing lip (not shown) of the seal mounted on an outboard end portion of the outer member 55 (FIG. 5), and various parts b to d extending from rolling surfaces to the radially inwardly stepped area are formed with a surface hardened layer 63 by means of an induction hardening technique or the like. Also, the serrations 64 formed on the inner peripheral surface of the hub unit 51 are formed with a surface hardened layer 65. Thanks to those surface hardened layers 63 and 65, the rolling fatigue lifetime, the frictional wear resistance and the rotational bending fatigue strength required in those parts a to d can be increased. (See, for example, the Patent Document 1 listed below.)    [Patent Document 1] Japanese Laid-open Patent Publication No. 2002-087008 (Pages 4 and 5, and FIG. 2)    [Patent Document 2] Japanese Laid-open Patent Publication No. 2005-003061
It has been found that although in those conventional wheel support bearing assemblies, formation of the surface hardened layer 62 in the corner area 61 of the wheel mounting flange 58 in the hub unit 51 is effective to increase the strength of the hub unit 51 and, at the same time, to reduce the weight thereof, without altering shape and dimension of the wheel mounting flange 58. However, an unexpected problem tends to arise that the wheel mounting flange 58 comes to be susceptible to deformation under the influence of heat treatment during the induction hardening process, resulting in surface runout of a brake rotor mounting surface 58. Reduction in wall thickness of the wheel mounting flange 58 also constitutes a cause of that tendency. The surface runout referred to above in turn results in brake judder under the influence of wobbling of the brake rotor 57 (FIG. 5), thus leading to reduction in automobile drivability and driving feeling. Therefore, it may be contemplated to eliminate the surface runout by additionally performing a turning process on the brake rotor mounting surface 59 after the heat treatment of the hub unit 51 to compensate for the deformation. However, the presence of a difference in hardness between the corner area 61, which has been hardened, and the brake rotor mounting surface 59, which has not been hardened, involves such a problem that a slight step tends to be formed at the boundary between the brake rotor mounting surface 59 and the surface hardened layer 62 of the corner area 61. Also, formation of the surface hardened layer 65 in the serrations 64 formed in the inner peripheral surface of the hub unit 51 tends to lead to a problem that as a result of thermal deformation of some or all of the serrations 64, a proper clearance (a close fit or a clearance fit) can no longer be secured between the serrations 64 and the mating, complemental serrations in the stem portion of the constant velocity universal joint 56 (FIG. 5).
In order to alleviate the foregoing problems, the applicant or assignee of the present invention has suggested such a method (the Patent Document 2 listed above) that in a wheel support bearing assembly of the type, in which the wheel mounting flange is formed integrally with one of the outer member and the inner member, at least one of the outer and inner members, which has the wheel mounting flange formed therein, is thermally refined. However, demands have been arisen for the hub unit to have the increased strength and durability under rotational bending conditions.
The present invention has been devised to substantially eliminate the various problems and inconveniences inherent in the conventional art and is intended to provide a wheel support bearing assembly, in which the strength and the durability of the hub unit under rotational bending conditions are increased and, at the same time, the weight thereof is reduced without altering the shape and dimension of the wheel mounting flange or without deteriorating the surface runout, and also to provide a method of making such wheel support bearing assembly.